The present invention relates to a flexible composite pipe for transporting oil, water or gas.
Iron pipes are mainly used for transporting oil or gas from submarine oilfields. However, flexible composite pipes are not receiving much attention due to easy laying.
For example, one type of flexible composite pipe is commercially available from Coflexip Corp, France. The composite pipe has a nylon inner pipe, Z-shaped reinforcing strips of small pitch and flat reinforcing strips of large pitch wound around the inner pipe, and a plastic outer sheath. This type of composite pipe is used for transporting crude oil or gas from many submarine oilfields. However, the temperature of oil flowing out from an oil well frequently exceeds 100.degree. C. When a composite pipe of this type is used at such an oilfield site, oil leakage occurs within a short period of time from pipe laying.
In conventional nylon inner pipes, 100.degree. C. is considered as the upper temperature limit, and a long pipe life cannot be guaranteed if the pipe is continuously exposed to this level of temperature. When oil to be transported contains water, even if the oil temperature is reasonably low, hydrolysis of the nylon resin occurs and the resin is degraded within a short period of time.
Heat-resistant resins to replace nylon resins generally include fluorine containing plastics and various engineering plastics. However, none of such resins satisfies all the requirements as to elongation, stress cracking and the like in order to manufacture high-temperature, high-pressure flexible transport pipes.
More specifically, polyvinylidene fluoride resins are known to have excellent extrudability, heat-resistance and chemical resistance and are used for pipe lining and for solid pipes. However, such resins have not been applied in the manufacture of flexible pipes, excluding extremely small diameter pipes.
The reason can be attributed to the high rigidity of polyvinylidene fluoride resins. When such a resin is used for purposes of the present invention, as the pipe is cut with a saw or the like in a bent state or in a straightened state after having been bent, cracks run for several tens of meters, particularly in wintertime, due to low temperatures. For this reason, polyvinylidene fluoride resins cannot be used in practice.
Cracking of polyvinylidene fluoride resins is also considered to be attributable to a high molding shrinkage of the resins and resultant large residual strain of a formed body, in addition to inherent high rigidity. In order to confirm this consideration, a pipe was manufactured by extrusion coating a polyvinylidene fluoride resin directly on a flexible interlocked metal pipe obtained by interlocking metal strips. When a notch was formed in the pipe, crack propagation from the notch was observed. It was demonstrated as a result of this test that cracks were formed from the recessed portions (projections in the resin formed body) at the engaging portions of the metal strips of the interlocked pipe.